This relates to integrated circuits and, more particularly, to programmable integrated circuits.
Programmable integrated circuits are a type of integrated circuit that can be programmed by a user to implement a desired custom logic function. In a typical scenario, a logic designer uses computer-aided design (CAD) tools to design a custom logic circuit. These tools help the designer implement the custom logic circuit using the resources available on a given programmable integrated circuit. When the design process is complete, the CAD tools generate configuration data files. The configuration data is loaded into programmable devices to configure them to perform the desired custom logic function.
A conventional programmable integrated circuit typically includes thousands of lookup tables (LUTs). Each LUT is implemented using multiple stages of multiplexers. As an example, a 4-input LUT may include four 4-input multiplexers in a first stage, two 2-input multiplexers in a second stage, and one 2-input multiplexer in a third stage. Each 4-input multiplexer has a first input terminal that is connected to an output terminal via a first inverter and a first transmission gate, a second input terminal that is connected to the output terminal via a second inverter and a second transmission gate, a third input terminal that is connected to the output terminal via a third inverter and a third transmission gate, and a fourth input terminal that is connected to the output terminal via a fourth inverter and a fourth transmission gate. The four transmission gates are controlled by various combinations of true and complementary versions of two control bits such that only one of the four transmission gates is turned one at any point in time.
In newer process technology nodes, the transmission gates are sometimes implemented using pass transistors that suffer from increased leakage. Keeping the current design of the LUTs would therefore lead to an unacceptable increase in power consumption. It is within this context that the embodiments described herein arise.